1999 Chevy Silverado runnning rich

I have a 99 truck with a 4.8 and it is running rich on #4 cyl. I replaced all the plugs, a new set of wires, and a new coil as well as a new injector for that cyl. I get a 300(engine misfire) code and it appears to be flooded when you start it. But why would this affect just 1 cylinder. Any ideas? Thanks

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Wednesday, August 27th, 2008 AT 6:26 PM

11 Replies

RHALL77

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Is the spark plug wet or something. How do you know its rich on #4. Is #4 where the misfire is?

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Wednesday, August 27th, 2008 AT 6:35 PM

1999 CHEVY DUDE

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The plug is black and the others are clean. I replaced the plugs, coil, wires, injector, and the problem is still there. The previous plug was black on # 4 also. Only 1 cylinder black ( rich ).

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Wednesday, August 27th, 2008 AT 6:47 PM

KHOOPER

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It either has a bad spark plug or a bad igniter coil, each cylinder has it's own igniter coil connected directly to the spark plug, remove the black plastic cover to get to them, this is the most common problem, in some cases, but less common a bad fuel injector or low engine compression could be the cause but I would try just replacing the spark plugs to see if it helps if not replace the coil next.

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Wednesday, August 27th, 2008 AT 6:49 PM

RHALL77

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Did you check this with a scan tool. Is cylinder #4 misfiring? You might just have a vacuum leak at the intake manifold. Also what is your fuel pressure, you might be low on pressure or volume causing a p0300. One way to check is drive the vehicle and see what your fuel trims are when it is under a load using a scan tool

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Wednesday, August 27th, 2008 AT 7:05 PM

1999 CHEVY DUDE

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I do not have a scan tool that will give me this info. But if it was a vacuum leak at or around #4 the plug would not be black (rich). The same situation with fuel pressure. The pressure would effect all cyl. Not just one. The only plug that is black is #4. All of the others are burning clean.

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Wednesday, August 27th, 2008 AT 7:15 PM

RHALL77

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Ok, well then does your truck burn oil?

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Thursday, August 28th, 2008 AT 7:10 AM

1999 CHEVY DUDE

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No, it uses just a bit but not an abnormal amount

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Thursday, August 28th, 2008 AT 9:32 AM

RHALL77

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1.

2.

whick plug does it look like

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Thursday, August 28th, 2008 AT 11:54 AM

1999 CHEVY DUDE

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#2, the picture that you have of #1 is a oil control problem (rings/. Valve seals)and that is not what I am dealing with. My situation is #2, a rich condition. Thanks

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Thursday, August 28th, 2008 AT 1:47 PM

RHALL77

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Check your thermostat and make sure you are running at a good operating temperature. Do you have coil over plug. Check for terminal tension make sure you have good contact. Or do you have a distrubitor and the timing is off. Stupid question, did you check your fuel pressure regulator, is it leaking. You fixed everything else, theres not much more it could be. Did you install factory plugs back in it.

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Thursday, August 28th, 2008 AT 5:26 PM

JOHN SHY

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My number 4 was doing the same thing on a 2000 silverado 5.3L. I pulled the vacuum tube off of the fuel regulator ( on the drivers side of the engine) and gasoline came out. The diaphram had ruptured and was sucking raw gas into the intake manifold. It repeatedly fouled plug #4 and I kept changing ignition parts. Good Luck!